1. Field of the Invention
The present invention relates generally to a handover process in a mobile communication system, and in particular, to a method of assigning to a base station (BS) a BS identification (ID) code when a mobile station (MS) is to select a site for handover.
2. Description of the Related Art
Once a MS enters a handover region, the MS implements a handover process by communicating with a plurality of adjacent BSs as well as with a serving BS. To implement the handover, the MS receives signals from the adjacent BSs, each of the BS transmit the same data to the MS. Inherent therein is interference which leads to deterioration of system performance.
To overcome the problems, an SSDT (Site Selection Diversity Transmit) scheme is employed for the handover. The SSDT scheme is the process in which the MS selects a BS considered the best in reception performance among the participating BSs(active set), notifies the selected BS to the participating BSs, and receives data from the selected BS only. The receipt of a signal from the selected BS only reduces the interference. In addition, since the MS selects the BS from which the MS receives a signal with high signal strength, the time required for the handover is decreased.
FIG. 1 illustrates an SSDT procedure. Referring to FIG. 1, while a BS is communicating with a MS, a BS network(Radio Access Network) requests the MS to measure the strengths of signals received from adjacent BSs. The MS measures received signal strengths and informs the BS network of the measurements. The BS network implements a soft handover based on the measurements and determines whether to provide the SSDT function. Prior to implementation of the SSDT, the BS network sets an active set and informs the BSs of the active set that the SSDT has initiated and that they are in the active set. Upon receipt of confirmations from the BSs, the BS network informs the BSs of an SSDT action time and the MS of the SSDT action time and the active set. Then, the BSs of the active set and the MS initiate the SSDT at the action time. The MS designates a BS whose signal is received with good quality as a primary BS and the other BSs in the active set as non-primary BSs. The MS encodes the ID number of the primary BS and transmits it to all the BSs in the active set so that the BSs are informed of the primary BS by decoding the ID code. The primary BS continues data transmission, while the non-primary BSs discontinue data transmission.
In a UMTS (Universal Mobile Telecommunications System) system, for example, the BSs receive the BS ID code on a reverse DPCCH (Dedicated Physical Control Channel) of the MS. A DPCCH frame is shown in FIG. 2. The DPCCH frame includes 15 slots, each slot having four fields, PILOT, TFCI (Transport Format Combination Indicator), FBI (Feed Back Information), and TPC (Transport Power Control). The BS ID code is transmitted via the FBI field.
An error may be generated when the primary BS decodes the ID code. The decoding error results in discontinuation of data transmission in the primary BS because it does not know that it has been designated as a primary BS. If the non-primary BSs are informed accurately as to what BS was designated as a primary BS, they also discontinue data transmission. Consequently, all the BSs in communication with the MS discontinue data transmission, thereby dropping a call.
Accordingly, it is very important to design an ID code representative of a primary BS in the SSDT operation to select one BS to communicate with in a handover region. AWGN (Additive White Gaussian Noise) is taken into account in designing ID codes currently. In this environment, the ID code is designed based on Hamming distance related with the whole code length.
The problem of the conventional ID codes is that they were designed in the AWGN environment only. The ID codes do not show good performance in a real mobile communication environment that also experiences fading, which implies that designing of the ID codes based on the Hamming distance of the whole ID codes is not appropriate. Without fading, it can be assumed that ID codes susceptible to decoding errors are distributed with a uniform probability in position. However, this is not the case when fading occurs. Therefore, it is preferable to design ID codes with due consideration given to fading channels. Besides, an active set must be updated frequently to implement a handover more efficiently in the mobile communication system.
It is, therefore, an object of the present invention to provide a method of generating a BS ID code representative of a primary BS taking fading into account in a mobile communication system.
It is another object of the present invention to provide a method of selecting a BS as a primary BS using an ID code with less decoding errors involved.
It is a further object of the present invention to provide a method of reassigning BS ID codes to adapt to a change in an active set when a MS measures the strengths of signals from a plurality of BSs and selects a BS considered best in reception quality for handover in a mobile communication system.
It is a further another object of the present invention to provide a new method of signal transmission process for SSDT performance improvement.
To achieve the above objects, a method of signal transmission in a mobile communication system having a mobile station communicates with a serving base station and a plurality of adjacent base stations, and a controller for controlling the base stations, comprising the steps of measuring reception strengths of signals received from the base stations and reporting the received signal measurements to the serving base station by the mobile station, reporting the received signal measurements to the controller by the serving base station, determining active base stations based on the received signal measurements, each active base station ID numbers and an active set category of the active base station, and transmitting the ID numbers to the base stations when there is category change of the active set by the controller, transmitting the base station ID numbers to the mobile station by the serving base station, selecting a base station which have the most high reception power level and generating ID code corresponding ID number of the selected base station by the mobile station.
To achieve the above objects, a method of providing site selection diversity in a mobile communication system having a mobile station communicates with a serving base station and a plurality of adjacent base stations, and a controller for controlling the base stations, comprising the steps of measuring reception strengths of signals received from the base stations and reporting the received signal measurements to the serving base station by the mobile station, reporting the received signal measurements to the controller by the serving base station, determining a category of a current active set of the mobile station based on the received signal measurements by the controller, comparing the category of a previous active set with the category of the current active set by the controller, reassigning an ID number of a base station retracted from a previous active set to a different base station in the current active set by the controller, if the category of the previous active set is greater than the category of the current active set, transmitting the reassigned base station ID number to the different base station and the category of the current active set to the base stations in the current active set by the controller.